I love photo groupings. They're a great way to display an art collection or many family portraits in a cohesive way. However, groups can look haphazard unless you have a big picture (excuse the pun) in mind. In our last apartment, I hung small frames at the same height all around our living room to make a border. During that little escapade, rather than rely on trial and error and risk the security deposit by hammering hundreds of holes in the wall, I had to devise a way to make sure 30 different types, sizes, and styles of frames would all hang at exactly the same height. I've since seen variations of my technique in several magazines (where were you, Blueprint, the winter of 2004 when I needed you?), but I've picked up some additional helpful tricks.
First, make a paper template of every frame for the grouping. Newspaper works well for this, or any cheap paper laying around. Make sure to label each template so it can be matched up later with the correct frame, hence the 'moose' label in the photo above.
Flip the frame face down and place the template over it. Use your fingers and feel for the picture hanger. Mark this spot on the paper template. If the frame has a wire to hang it, gently push the wire up like it would hang if the picture was on the wall and mark the top of the wire curve.
Grab some tape and all the paper templates. Tape the templates up on the wall where the pictures will hang. Pay attention to the spaces between the frames. The grouping will look best if these spaces are the same all around. Move the templates around until you're happy with the layout. If you think you're going to rearrange the templates quite a bit, I'd recommend using blue painter's tape to avoid damaging the walls. It also stays stickier than regular tape after several template moves.
Once you've settled on the layout, pull out the hammer and nails and pound a nail in at the spot you marked on each template. The nail will be right where it needs to be for that frame's picture hanger, guaranteeing each picture will hang in exactly the same spot as the template. If any of the frames are really heavy, drywall anchors are probably a better way to hang them. In that case, just drill a hole for the anchor at the spot marked on the template.
Rip off the templates and hang each picture in the perfect spot. Then stand back and admire your handiwork. That was hard work, you probably deserve a drink. I know I did after I finished the wall below.